If you had bothered to look at the financials, you would see that as things stand today, the consumer OS sales do not really make that much money. The vast majority of MS's money comes from (and these licensing costs are not changing):
* Enterprise OS licenses
* OEM Licenses
* Server and Application Licenses
* Services (Azure, etc..) fees
* Hardware (Keyboards, Mice, Xbox, etc..)
Retail OS Upgrades do not account for much in the way overall revenues... so this "giveaway" isn't going to cost them much, and the benefit is that they get all those old systems upgraded.